1. Field of the Invention
The invention relates to the attachments that attach blades to a rotor and more specifically to a method for extending their life, this method employing a particular embodiment of the xe2x80x9cultrasonicxe2x80x9d peening technique.
2. Technological Background
In an aerojet engine, the bladed rotors traditionally consist of a rim, at the periphery of which a number of removable blades are mounted. The mounting device is known in this patent application as a xe2x80x9cblade attachmentxe2x80x9d. This device comprises a dovetail groove machined in the rim and a root also in the shape of a dovetail machined at the base of the blade, assembly being by fitting the root into the groove. In a more elaborate embodiment known as the xe2x80x9cChristmas tree rootxe2x80x9d embodiment, the dovetail has several xe2x80x9cbulbsxe2x80x9d of decreasing size, typically three bulbs, each bulb separately performing the function of the dovetail. In what follows, the term xe2x80x9cdovetail attachmentsxe2x80x9d encompasses these two forms of attachment. The blade roots are fitted into the grooves by sliding with limited clearance, the roots then being immobilized without clearance by various locking means. It will be understood that the grooves and the blade roots are the site of high stress concentrations and that they therefore have to be produced with particular care. Turbojet rotors are conventionally made of steel, titanium alloy or a nickel or chromium-based superalloy.
Conventionally, rotors undergo peening by projecting small beads made of a hard material using one or more compressed-air nozzles. This peening has the effect of creating compressive prestress at the surface of the rotor over a depth of a few tenths of a millimeter, this prestress delaying the onset of cracks resulting from the high stresses and thus extending the life of the rotor. If need be, peening is preceded by a heat treatment of the part to be treated in order to relieve the residual stresses that remain in this part. When certain portions of the part do not need to be peened, they are customarily protected by coating them with a material, such as an elastomer, that has sufficient hardness to resist the impact of the beads.
Intensive peening with an Almen intensity of the order of F15A to F17A and making it possible to create a compressive prestress of the order of 900 to 1100 MPa (mega pascals) at the surface of the rotor is desirable, these rotors customarily being made of steel, of titanium alloy or of chromium-based or nickel-based superalloy. Unfortunately, such peening greatly increases the roughness of the treated surfaces and thus reduces the resistance to wear by vibrational friction of the surfaces of the grooves and of the blade roots which are in mutual contact.
This increase in the roughness of a surface undergoing intensive peening by the blasting of small beads is backed by various documents:
Patent EP 0 922 532 paragraph [0005] column 1, lines 33 to 38. One of the recommended solutions is to reduce the intensity and coverage of the peening, paragraph [0006], lines 39-40. This same patent indicates at line 41 that this may result in a reduction in the life of the part. In the magazine xe2x80x9cSouder [Welding]xe2x80x9d No. 5 of September 1998, the study xe2x80x9cLe principe du choc laser et ses applications au traitement des matxc3xa9riaux [The principle of laser shock and its applications to materials treatment]xe2x80x9d makes a comparison between xe2x80x9claser shockxe2x80x9d peening and conventional peening and indicates in the penultimate paragraph of page 13 that conventional peening creates microcraters resulting from the impacts of the beads and increasing roughness. According to the examples given in the first table on page 14, the roughness (Ra) of a machined surface can increase from 2.3 xcexcm to 5.5 xcexcm after intensive peening
The article xe2x80x9cLe grenaillage de prxc3xa9contrainte [Prestressing peening]xe2x80x9d, published in 1992 by CETIM page 105-123 reports a national conference held on Sep. 25 and 26, 1991 at Senlis in France. In the penultimate paragraph of page 108, it indicates that the peening of a machined surface leads to an increase in the roughness value. This same article specifies, in the last paragraph of this same page, that the roughness can be reduced by performing the peening in several passes of decreasing intensity. It should be understood that intensive peening to start with increases the roughness and that the increasingly lighter peening operations which follow reduce the roughness by evening out the peened surface. This solution does, however, have the disadvantage of being lengthy because several peening operations are needed, the first prestressing the peened surface and the next peening operations gradually reducing the roughness which appeared during the first peening operation.
The problem to be solved is that of simultaneously increasing the fatigue strength and the resistance to vibrational friction of the rotors at the blade attachments, it being necessary for this increase not to lead to an appreciable increase in the time and cost involved in manufacturing the rotors.
Peening by blasting beads is currently experiencing a new embodiment known as xe2x80x9cultrasonic peeningxe2x80x9d where the beads are no longer blasted out of a nozzle by a jet of compressed air but by the percussion against these beads of the surface of a sonotrode vibrating at frequencies of the order of 20 to 60 kHz, the beads being kept inside a chamber, the part to be peened being, depending on its dimensions, immersed inside the chamber or offered up to an opening of this chamber.
Patent FR 2 743 742 discloses an application of ultrasonic peening to cookware allowing the microcavities created beforehand on the surface of the utensil to be reduced to encourage the adhesion of a coating to part of the utensil. This patent indicates, on page 5 line 32, that the cookware is made of aluminum. It is known that that material is soft, and its prestressing does not exceed 150 to 200 MPa. It is very much below the desired prestressing of 900 to 1100 MPa. That patent also indicates on page 1, line 31 that the surface obtained is smooth, but specifies on page 5, line 14 that the peening (which it refers to as xe2x80x9cbillagexe2x80x9d) lasts from 0.5 to 5 seconds. Even for a soft material, this then is only a very light surface peening that is in no way comparable with the intensive compressive prestressing peening operations carried out on aeronautical parts, the surfaces of these parts having to be exposed to the peening for a duration of 4 to 10 minutes at least. That patent therefore provides no solution to the problem that is to be solved.
According to the invention, for a rotor blade assembly comprising a rotor and a plurality of blades removably attached to said rotor, said rotor comprising a rim having a periphery to which each of said plurality of blades is attached by attachments comprising first and said second components, said first component comprising a dovetail groove arranged in said periphery of said rim, and said second component comprising a root formed on said blade, said root being of a shape that complements said groove and being able to be fitted into said groove so as to attach said blade to said rim, there is provided a method for extending the life of said attachments by peening a surface of at least one of said first and second components so as to create compressive prestress at said surface, said method comprising the steps of:
providing a sonotrode which is adapted to be vibrated ultrasonically;
forming a chamber which is delimited at least partly by said sonotrode and the surface which is to be peened;
providing a plurality of beads in said chamber; and
vibrating said sonotrode whereby said beads are mobilized in said chamber by the percussion of said sonotrode so that said beads are projected against said surface to effect untrasonic peening of said surface with an Almen intensity at least equal to F8A.
The inventors have found that intensive ultrasonic peening only slightly increases the roughness of the treated part, unlike conventional peening employing a nozzle with a jet of compressed air. The invention thus puts this unexpected property to use to increase the fatigue strength of the blade attachments while at the same time maintaining good resistance to wear by vibrational friction.
One advantage of the invention is that it also increases the resistance to wear by vibrational friction of the blade attachments, because the high compressive prestressing of the surfaces of the components of the blade attachments causes them to harden by work hardening. Advantageously, use will be made of beads with a diameter at least equal to 0.8 mm so as to improve the effectiveness of the peening and to stabilize or even reduce the roughness of the treated parts.
Advantageously, the compressive prestress will be at least equal to 500 MPa.
According to one particular embodiment of the invention the method is applied to xe2x80x9caxialxe2x80x9d grooves on the rims of bladed rotors, wherein said dovetail grooves are arranged axially in said periphery of said rim, said axial grooves being approximately straight and open at each end, and wherein said sonotrode is capable of being introduced into said grooves and includes means for sealing said sonotrode in said grooves and two wings capable of covering said open ends of said grooves and of closing them off with a clearance smaller than the diameter of said beads, said method further comprising the steps of:
arranging said rim above said sonotrode in an appropriate position for bringing each axial groove over said sonotrode by rotating said rim about its geometric axis,
arranging said plurality of beads on said sonotrode,
turning said rim to bring each axial groove in turn over said sonotrode, each axial groove then being subjected to the steps of:
covering said open ends with said wings and bringing said sonotrode into said axial groove,
peening said axial groove by setting said sonotrode in vibration,
withdrawing said sonotrode.
According to another particular embodiment of the invention, the method is applied to xe2x80x9cannularxe2x80x9d grooves on the rims of bladed rotors, wherein said dovetail grove is arranged annularly in said periphery of said rim, said annular groove including a mouth, and a local opening for insertion or removal of said roots of said blades, and wherein said sonotrode is capable of being introduced into said mouth of said annular groove and includes means for sealing said sonotrode in said mouth, and two wings are capable of passing into said annular groove with a clearance smaller than the diameter of said beads, said method further comprising the steps of:
arranging said rim over said sonotrode in an appropriate position for causing said annular groove to travel over said sonotrode by rotating said rim about its geometric axis,
arranging said plurality of beads on said sonotrode,
presenting said local opening over said sonotrode,
bringing said sonotrode and said two wings into said local opening, said sonotrode being level with said mouth and aligned with said mouth, said two wings lying one on each side of said sonotrode and aligned with said axial groove,
turning said rim and setting said sonotrode in vibration when said two wings and said sonotrode are in said annular groove,
stopping said sonotrode as soon as a wing comes out in said local opening,
stopping rotation when both wings and said sonotrode are in said local opening.
According to another particular embodiment of the invention, the method is applied to the roots of the blades, wherein use is made of a chamber including a bottom, said bottom having an opening through which said sonotrode passes with a clearance smaller than the diameter of said beads, said chamber being covered by a thin lid, said lid having a number of openings of a shape that complements said roots, the distance between said lid and said sonotrode being at least equal to the height of said roots, said method further comprising the steps of:
introducing said plurality of beads into said chamber,
introducing said roots into said openings of the lid and immobilizing said blades;
setting said sonotrode in vibration to carry out said peening.